Current methods of monitoring seismic waves must contend with a multitude of coherent noise sources. On land, standard practice is to couple geophones to the ground with a spike attached to their bases. The geophones receive noise signals from wind and any traffic which may be in the area. In marine seismic surveys, hydrophones are suspended in the water and register incoming signals. Included in these signals are noise trains from such sources as wind, wave motion, currents, marine life and vessels which may be in the vicinity of the hydrophone.
Presently there are very few methods for improving signal-to-noise ratios for seismic monitors. On land, the signal-to-noise ratio is sometimes enhanced by shallow burying of the geophones. This improves the acoustic coupling slightly while it reduces the noise level by partially isolating the geophones from noise sources. The success of this method is rather limited because of acoustic dispersion in the near surface. Additionally, shallow burial of geophones gives only slight isolation from environmental noise effects. Similar results are obtained in water with the use of ocean bottom seismographs or "bay phones". The geophone is coupled to the sea floor which is a better acoustic conductor than the water. Besides the improved signal because of improved coupling, the ocean bottom seismograph is further removed from wind, wave and vessel noise sources.
The techniques presented herein will give an order-of-magnitude improvement in the signal-to-noise ratio for seismic sensors. The concept is to implant the geophones in competent subterranean rock. By drilling a borehole into competent rock and then grouting the geophones into place, the signal will be greatly improved because of solid coupling to an acoustically conductive medium. Additionally, the sensors will be well insulated from environmental noise sources. This isolation is accomplished in part because the dispersive surface above the geophones will insulate them from surface noise sources.
With the better signal-to-noise ratio, this technique has applications beyond the seismic exploration for oil and gas. The increased sensitivity of the system will allow for acoustic tomographic monitoring of oil or gas depletion in hydrocarbon reservoirs. Similarly, the techniques can be used in monitoring seismic events or micro-seismic activity associated with enhanced oil recovery operations, hydrocarbon depletion caused subsidence or overburden subsidence due to underground mining operations. The implanted geophones can also be used in rock quality and progress monitoring of underground construction.
Implanted geophones also can be used as remote seismic detectors. Such seismic monitoring can provide early warning of earthquakes or monitor nuclear explosions for test yield restrictions. With the use of amplitude measurements and triangulation within an array of geophones, the magnitude of a seismic event and its location can be determined. The techniques can also be used for permanent monitoring adjacent to open pit mining or any other situation which employs blasting operations. Such monitoring can be for environmental compliance or for forensic reasons.
These applications mentioned for drill- and grout-geophone implanting are not by any means all inclusive. Rather, they are mentioned to give a perspective of the wide range of applicability for grouting geophones into boreholes.